Luckily not my car, and an answer isn’t needed quickly. I just happened to be behind one of these types of cars while coming home for lunch break, and (since I drive primarily at night), I see these types of cars several times a night.
Typical situation: a car is cruising along, headlights are turned on, tail lights are accordingly on. When the car comes to a stop at a red light, the tail lights will go dark… usually the third brake light will come on, but often all rear lights are dark. They come back on when the brake is released.
What kind of wiring problem would cause something like this? The lights obviously work in some fashion… what could get messed up such that braking would break the circuit?
Question… have you noticed whether or not the cars have ‘after-market’ type, tail lights?
I would hazard a guess that they do, and that said lights weren’t properly installed. I’ve noticed the same/similar conditions on cars in my area.
IIRC, they looked like they had non original tail lights.
Either that, or just ‘wonky connections’.
Rear car lights typically use twin filament bulbs, usually P21/5W or some variation.
The symptoms the OP describes usually happen when the two filaments inside the bulb come into contact. The solution is very simple: replace the bulbs. They are dirt cheap.
Other symptoms might be that the front parking lights are always on, indications on the dashboard go awry, etc.
The filaments contacting each other (other than at their ground side, where they are already in contact) is exceedingly rare and will not cause the symptom described.
This will not cause the taillights to go out when the brakes are applied.
This is the most likely cause of the described symptom.
Braking doesn’t break the circuit. The normal circuit is already broken. Braking applies voltage to a part of the circuit that was acting as a substitute ground; when the taillight filament has 12v on both sides of it then it ceases to light up.
How does the ground work through the brake light circuit? I’m envisioning that the brake light is attached to +12V via a momentary switch. When the switch is closed, one side of the brake light has 12 volts (the other side is hopefully grounded). When the brake light switch is open, it’s left floating so that there would be no path for the tail light current to travel, to ground or otherwise. Am I missing something?
It’s the third (high center) brake lamp that allows this. If the ground for the corner (left and right) tail/brake lamps is open, the current for the taillights travels through the left and right brake lamp filaments and then through the third brake lamp filament to its ground. I would expect the third brake lamp filament to glow dimly, but probably so dimly as not to be noticeable.
Gary T nailed it. It’s a bad ground. Probably from a poor body shop repair.
I once had a rental car that when you stepped on the brakes the dash lights and radio display went out. Looked in the trunk, yup ground wires not connected.
All problems of this nature are known as ‘a short’. They aren’t necessarily caused by a short, but when something is wrong with a car’s electrical system it’s always called ‘a short’. So you have a short, which means nothing, maybe even the opposite of a short. Luckily you’ve received some good information here about the actual problem since it can cost a lot of money to fix ‘a short’.
You’re both right. This one appears to be an open (the opposite of a short), but laymen tend to use the term “short” instead of the more accurate and clunkier “wiring problem.”
Bulbs will often have grounds that are riveted instead of soldered or they will use one small connector to ground several lights and it will start to build up resistance and eventually stop making contact. Bad grounds can cause some screwy looking problems sometimes.
